A dual-rotor type motor having rotors both inside and outside a stator can increase an amount of magnetic flux by capitalizing on the two rotors, thereby producing greater torque although the motor is compact size. This motor is hereinafter referred to as a dual rotor motor. The maximum rpm of this motor; however, is obliged to be lower under the condition of the power supply voltage being kept constant because an induction voltage proportional to magnetic flux inter-linking with the windings becomes higher. To obtain a greater range of a high rpm operation, an electric current for canceling the magnetic field of permanent magnets is used besides a torque current component that is used for generating torque, whereby the number of inter-lining magnetic fluxes can be reduced. This electric current is referred to as “field weakening current”, and this control method is referred to as “field weakening control” which is generally practiced in the industry. However, copper loss increases substantially at a greater amount of the field weakening current. If the motor produces a greater copper loss than iron loss, this field wakening control method will substantially lower the efficiency of the motor.
To achieve both of great torque at a low speed and a high rpm by the dual rotor motor, a technique of reducing the inter-linking magnetic fluxes by a mechanical method has been proposed instead of the field weakening control method. This mechanical method is disclosed in, e.g. Patent Literature 1, which proposes that the relatively fixed angle between an inner rotor and an outer rotor of a vehicle synchronous machine be changed step by step. To be more specific, multiple screw-holes are provided to a frame connecting section of both the rotors so that combinations of fastening screws can be changed for changing the relative angle between the inner rotor and the outer rotor. This structure allows varying the relative angles between both the rotors in response to models of a vehicle or necessity before shipping the vehicles. As a result, the vehicle synchronous machines of a greater number of types can be manufactured with a fewer models, and the loads to manufacturing and maintenance can be thus alleviated.
Another synchronous machine employing a dual rotor motor is proposed. A second rotor can be rotated relative to a first rotor within a given angle range, and they cannot be relatively rotated outside the given angle range, namely, this machine includes a mutually regulating mechanism for rotors. This proposal is disclosed in, e.g. Patent Literature 2. This mechanism allows controlling both of the rotors to stay at either one of two steady angle positions where the rotors can relatively and steadily rotate, and combined field magnetic fluxes of different sizes inter-link with the armature coil at these two steady positions. This method is suitable for a generator motor linked to a car-engine that needs great torque at a low rpm and uses a wide range of rpm.
The synchronous machine disclosed in Patent Literature 1, i.e. a relatively fixed angle between the inner rotor and the outer rotor can be varied step by step by changing combinations of screw fastenings before the machine is shipped. This machine is thus not good for an application of frequent changes between two operation modes, namely, great torque at a low rpm and a high rpm.
The synchronous machine disclosed in Patent Literature 2, i.e. it has a mutually regulating mechanism for rotors, and this mechanism allows the second rotor to rotate relatively to the first rotor within a given angle range, and it prohibits the relative rotation outside the given angle range. Since this synchronous machine controls two steady angle positions in a rotating direction and counter-rotating direction, it is suitable for a generator motor rotating in a single direction; however, it cannot be used in an application of rotating in either directions.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2002-359953    Patent Literature 2: Unexamined Japanese Patent Application Publication No. 2002-165426